1. Field of the Invention
This invention relates to an apparatus for forming optical elements such as lenses, prisms or filters, and more particularly to the molds thereof.
2. Description of the Prior Art
Heretofore, optical elements such as lenses, prisms or filters have often been manufactured by the glass polishing process. However, the polishing process requires a considerable time and skill. Also, to manufacture aspherical lenses by the polishing process, a higher polishing technique is necessary and the processing time unavoidably becomes long. In contrast with such an optical element manufacturing method using the polishing process, there is a method of manufacturing optical elements by the molding using heating and pressurization. (See U.S. Pat. Nos. 3,833,347; 3,900,328; 4,139,677; etc.) According to this molding method, optical elements can be manufactured within a short time and even aspherical lenses, like spherical lenses, can be manufactured easily and within a short time, but problems are still left to be solved in this molding method using heating and pressurization. One of the problems is the improvement of decentering. In the case of spherical lenses, even if there is decentering thereof, alignment can be effected in the post-process, while in the case of aspherical lenses, alignment cannot be effected and consideration must be given to the structure of the mold.
Heretofore, molding has been effected by a structure comprising a fixed mold, a lower mold and an upper mold as shown in FIG. 1 of the accompanying drawings and a mating portion has been provided between the upper mold and the lower mold although there has been more or less modification thereof, and there has been a disadvantage that the accuracy of mating directly affects the accuracy of decentering. That is, in the apparatus of the prior art shown in FIGS. 1(a) and (b), the design is such that a mass of heated glass is placed on a lower mold 2 held by a fixed mold 1 and the spherical surface of a lens is formed by pressing an upper mold 4 against the lower mold 2 by means of a hydraulic cylinder (not shown). The upper mold 4 of this prior art apparatus mates with the inner surface 1a of the fixed mold 1 and is designed to be moved along a predetermined axis 0.sub.1 -0.sub.2, but the presence of the sliding mating portion 1a between the fixed mold 1 and the upper mold 4 makes it difficult to make the axis of the lower mold completely coincident with the axis of the upper mold. Due to the incoincidence between these axes, in the case of a lens 6 as shown in FIG. 2 of the accompanying drawings, the centers of sphere a.sub.1 and b.sub.1 of the functional surfaces 6a and 6b of the lens are not coincident with the optic axis and thus, the lens thicknesses T.sub.1 and T.sub.2 become different from each other and decentering occurs in the lens 6.
As described above, the prior art apparatus suffers from the problem that decentering of the lens is liable to occur and further, in the apparatus using press molding with the upper and lower molds as previously described, it is necessary to set with high accuracy the pressing position of the upper and lower molds in a case where the functional surfaces of the optical element are aspherical, and there are many problems still left to be solved such as the design of the molds and complication of the structure of the apparatus.